Concept of Watershed Watershed is a natural hydrologic
entity that encompasses a ‘specific area’ stretch of land surface where from
rainfall or run off flows to a ‘specific defined drain’ be it a channel/nullah,
small stream or river. Watersheds are
considered appropriate units for both surveys and assessment of soil and land resources as
well as for planning and implementation of various developmental programmes. Owing to optimum interaction and synergetic effect
of land and water resources within a watershed, watershed approach, over the period of
time, has become popular in various developmental programmes in the country. The process of delineation of
watersheds has a scientific and rational approach. However, for presenting them as units
for planning and development, formulation of an appropriate framework for watersheds is a
pre-requisite. Such framework shall not only follow a hierarchical system of delineating
bigger hydrologic units into watersheds but also include a codification system so that
each watershed or units of interventions can be identified as an ‘unique’
individual entity with clear identifiable linkage with higher hierarchical units, that is, basin, catchments, subcatchment. The concept of stream order is
often followed in geomorphic analysis of natural drainage system. However, framework of
watersheds requires a different approach also indicating macro and micro level of
delineation so that programmatic interventions units are of manageable sizes. Each of the
big drainage systems needs to be divided and sub divided through stages. The framework applied in this
document employs popular terms like basin, catchment, subcatchment, watershed for denoting
various stages of delineation from macro to micro levels. Throughout this document, size of
hydrologic entities is governed by the size of the stream or river or the point of
interception of the river like a dam, barrage, etc. For example, the size of the Mahanadi
river basin is 145 lakh ha whereas the size of the catchment of Hirakud dam on Mahanadi is
83 lakh ha. Size of sub-watersheds is,
however, delineated keeping in view of its relative importance in land and water resource
development programmes. On the other hand,
size of smallest hydrologic units which are below sub-watersheds are generally governed by
viability and working feasibility of implementation of various developmental programmes. |